Sintering and dielectric properties of fine-grained BaTiO3 ceramics

A new sintering method, spark plasma sintering (SPS), is described for the sintering of fine-grained BaTiO₃ ceramics. Dense ceramics with fine grain size of near 170 run are obtained using SPS at low temperature of 900°C without holding time. The dielectric measurements of various grain size specimens show that the dielectric constants decrease with the reduce of grain size, and diffuse phase transition showed in the dielectric-temperature spectra. The variation in dielectric properties with grain size is explained.     

Effect of ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass addition on the microwave dielectric properties of BaO-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The effect of ZnO-B₂O₃(ZB) glass addition on the sintering behavior, microstructures and microwave dielectric properties of BaO-Nd₂O₃-TiO₂-Bi₂O₃ (BNTB) system was investigated with the aid of X-ray diffraction, scanning electron microscopy and capacitance meter. It is found that the ZB glass addition, acting as a sintering aid, can effectively lower the sintering temperature of BNTB system to 850 °C. The dielectric constant of BNTB-ZB ceramics increases with the increase of soaking time and the value of dielectric loss decreased with increasing soak time. The optical dielectric properties at 1 GHz of ɛ=74, tan δ=4×10⁻⁴, and TCC=25 ppm/°c were obtained for the BNTB system doped with 25 wt% ZB glass sintered at 850 °C for 2 h, representing that the BNTB-ZB ceramics could be promising for multilayer low temperature co-fired ceramics applications.     

Low-temperature sintering and microwave dielectric properties of Ba<Subscript>2</Subscript>Ti<Subscript>3</Subscript>Nb<Subscript>4</Subscript>O<Subscript>18</Subscript> ceramics

The effects of CuO and H3BO3 additions on the low-temperature sintering,microstructure,and microwave dielectric properties of Ba2Ti3Nb4O18 ceramics were investigated.The addition of less amount of CuO (< 1 wt%) considerably facilitated the densification of Ba2Ti3Nb4O18 ceramics.Appropriate addition of H3BO3 (< 3.5 wt%) remarkably improved the microwave dielectric properties of ceramics.The addition of H3BO3 and CuO successfully reduced the sintering temperature of Ba2Ti3Nb4O18 ceramics from 1300 to 1050 ℃.B...     

Influences of heat treatments on crystallization in SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-K<Subscript>2</Subscript>O-F glass-ceramics

Five kinds of heating treatment processing were chosen according to the experiment result of differential scanning calorimeter to prepare SiO2-Al2O3-MgO-K2O-F glass ceramics samples.The effects of heat treatment processing on the crystallization of these samples were explored by X-ray diffraction and scanning electron microscopy techniques.The results indicate that phase separations can occur in the bulk regions of the glass sample when holding at 670 ℃ for 3 h.The phase separation can accelerate the precip...     

Dielectric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics by impedance spectroscopic method

Various lead-free ceramics have been investigated in search for new high-temperature dielectrics. In particular, Bi₄Ti₃O₁₂ is a type of ferroelectric ceramics, which is supposed to replace leadcontaining ceramics for its outstanding dielectric properties in the near future. Ferroelectric ceramics of Bi₄Ti₃O₁₂ made by conventional mixed oxide route have been studied by impedance spectroscopy in a wide range of temperature. X-ray diffraction patterns show that Bi₄Ti₃O₁₂ ceramics are a single-phase of ferroelectric Bi-layered perovskite structure whether it is calcined at 800 °C or after sintering production. This study focused on the effect of the grain size on the electric properties of BIT ceramics. The BIT ceramics with different grain sizes were prepared at different sintering temperatures. Grain becomes coarser with the sintering temperature increasing by 50 °C, relative permittivity and dielectric loss also change a lot. When sintered at 1 100 °C, r values peak can reach 205.40 at a frequency of 100 kHz, the minimum dielectric losses of four different frequencies make no difference, all close to 0.027. The values of E _a range from 0.52 to 0.68 eV. The dielectric properties of the sample sintered at 1 100 °C are relatively better than those of the other samples by analyzing the relationship of the grain, the internal stresses, the homogeneity and the dielectric properties. SEM can better explain the results of the dielectric spectrum at different sintering temperatures. The results show that Bi₄Ti₃O₁₂ ceramics are a kind of dielectrics. Thus, Bi₄Ti₃O₁₂ can be used in high-temperature capacitors and microwave ceramics.     

Preparation and properties of crystallizable Glass/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites for LTCC material

The investigated low temperature Co fired ceramics(LTCC) composite of 60wt% CaO-Al2O3-B2O3-SiO2 glass and 40wt% α-Al2O3 as a filler is a non-reactive system, which is a critical part of the low temperature Co fired ceramics process. Through a study on densification process, the phase transformation and microstructure can be revealed. Its composites typically consist of CaO-Al2O3-B2O3-SiO2 glass and α-Al2O3 powders of average particle size (D50=3.49 mm). The sintering behavior, phase evaluation, sintered morphology, and microwave dielectric properties were investigated. In the fire range of 800 to 900 ℃, the composites were crystallized after completion of densification. It is found that the composites start to densify at 825 ℃, simultaneously, the dielectric constant (εr) reaches its maximum. With increasing heat-treatment temperatures, due to the loose microstructure of the material, tanδ increases slightly. The last of the sintered samples were identified as partly Anorthite at 850 ℃. At that temperature it has εr of 7.9 and tanδ less than 1×10-3, and can be used as a promising LTCC material.     

Manufacture Process of 8Y2O3 Stabilized ZrO2 from Nano Powders

The manufacture process of 8 mol% Y2O3 stabilized ZrO2 ( YSZ ) from nano powders, including the forming and sintering stages, was studied. During the forming process of YSZ powders, the relative density of YSZ increases lineally with the forming press, and the sintering linear shrinkage of YSZ to the forming press compiles to the parabola trend. When the forming press exceeding 500MPa, the samples with lower shrinkage and high density were obtained. The sintering temperature of YSZ decreases greatly because of the small size and high active surface of YSZ powders. As a result, the beginning sintering temperature of YSZ made in the experiment is as low as 825℃, and the end sintering temperature is 1300-1350℃ . The relative density of YSZ ceramic by solid sintering at 1300-1350℃ is more than 97% , with little and small pores in the uniform microstructure.     

Fabrication of Ti2AlC by Spark Plasma Sintcring from Elemental Powders and Thermodynamics Analysis of Ti-M-C System

A ternary-layered carbide Ti2AlC material could be synthesized by spark plasma sintering（SPS） technology using elemental powder mixture of Ti, Al and active carbon. By means of XRD and SEM, phases were identified and microscopically evaluated. The experimental results show that the main phase in the product was fully crystallized Ti2AlC with small particle size when sintered at 1200℃. The synthesis temperature of SPS was 200-400℃ lower than that of hot pressing （HP） or hot isostatic pressing （HIP）. Through thermodynamics calculations, the mechanism of Ti2AlC was studied by calculating changes of Gibbs free energy of reactions.     

Dielectric Properties of B2O3 Doped MgTiO3-CaTiO3 System Ceramics

The dielectric ceramics with a main crystal phase of MGTiO3 and additional crystal phase of CaTiO3 were prepared by the conventional electronic ceramics technology .the strucures of MgTiO3 are ilmenitetype,and belong to hexagonal syngony.the ratio of MgTiO3 to Ca TiO3 doping on the dielectric properties of MGTiO3-CaTiO3(MCT)ceranics were inrestigated.the addition of B2O3 decreases the sintering temperatnre and results in rapid desification without obrious negative effect on the Q values of the system(Q=1/tan ).B2O3 exists as liquid phase in the sintering process,promoting the reactions as a singering agent.     

Relationship between Liquid Phase Content and the Orientation of PMNT Ceramics by TGG Method

The effects of glass frit on the sintering and electric properties of PMN-PT textured ceramics were investigated. The glass frits, including PbO, Bi2O3 and ZnO, were selected since liquid phase sintering lowered the PMN-PT sintering temperature. The piezoelectric properties of PMN-PT ceramics with glass frit addition are strongly dependent on the densification. The addition of glass frits into PMN-PT matrix reduced the sintering temperature to 1 100 ℃ instead of 1 150 ℃ for samples without glass. The piezoelectric coefficients (d33) of PMN-PT textured ceramics achieved 568 pc/N with 1 wt% excess PbO.     

Phase structure and electrical properties of La<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped BiInO<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> ceramics with high curie temperature

The phase structure and electrical properties of pure and La2O3-doped Bi-InO3-PbTiO3 （BI-PT） ceramics were studied respectively. In （1 -x）BI-xPT （x=0.72-0.80） ceramics, the stability of tetragonal phase increased with increasing x, and pure perovskite structure was obtained for x=-0.80 ceramics. The phase transition temperature range was between 575 ℃ and 600 ℃ for x=0.72-0.80 ceramics, higher than that of PT （-490 ℃）. The c/a ratio almost linearly decreased with increasing La2O3 content in x-0.80 ceramics. It is believed that Pb^2＋ vacancies were formed by La^3＋ substituting Pb^2＋ in La2O3-doped BI-PT ceramics. Tc shifted to lower temperature by 30 ℃/mol% La2O3. The maximum dielectric constant 8557 around 559 ℃ was exhibited in 0.5mol%-doped BI-0.80PT ceramics. La2O3-doped ceramics could be poled resulting from decreasing of c/a ratio and improving of dielectric loss and resistivity. The maximum piezoelectric coefficient d33 was 12 pC/N for 2mol%-doped BI-0.80PT ceramics.     

Microwave sintering of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZrO<Subscript>2</Subscript>-WC-Co cermets

Composite powders of nanocrystalline WC-10Co (15wt%),Y2O3 (8mol%) stabilized nanocrystalline ZrO2 (30wt%),industrial cobalt powder (4.5wt%) and submicron Al2O3 (55wt%) composite powders were fabricated by high-energy ball-milling process.The nanocomposite powders were consolidated by microwave sintering process at temperature ranged 1300℃-1550℃ for 15min,respectively.The optimum consolidation conditions,such as temperature,were researched during microwave sintering process.Vickers Hardness of the consolidated cermets was measured by using a Vickers indentation test,and density of specimens was also determined by Archimedes’ principle.Microwave sintering process could not only increase the density of Al2O3-ZrO2-WC-Co cermets and reduce the porosity,but also inhibit abnormal grain growth.     

Preparation and microstructure of spinel zinc ferrite ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> by Co-precipitation method

Spinel zinc ferrites ZnFe₂O₄, prepared by co-precipitation method using the zinc nitrate Zn(NO₃)₂·6H₂O and ferric nitrate Fe(NO₃)₃·2H₂O as the raw materials, were characterized by the thermo gravimetric analysis (TG) and differential scanning calorimeter (DSC), X-ray diffraction (XRD) and scanning electron microscope (SEM). The influence of synthesis conditions, such as Zn/Fe molar ratio, pH value, the sintering temperature and time, on the microstructures was detailedly investigated. The relationships between the microstructures and the synthesis conditions were discussed. The results show that the pure spinel zinc ferrites ZnFe₂O₄ are formed when the Zn/Fe molar ratio is 1.05:2 at pH=8.5 or Zn/Fe molar ratio is 1:2 at Ph=9-10, and the precursors are sintered at 1100 °C for 4 h. Especially no other phases are observed when the Zn/Fe molar ratio is 1:2 at pH=10 and the precursor is sintered above 700 for 4 °C h. The higher sintering temperature and longer sintering time contribute to grain growth.     

Preparation of nanocrystalline BaTiO<Subscript>3</Subscript> ceramics

The high-dense nanocrystalline BaTiO₃ (BT) ceramics with grain size smaller than 100 nm have been successfully prepared by the two step sintering and the spark plasma sintering (SPS) process. The successive transitions in nanograin BT ceramics from rhombohedral to orthorhombic, tetragonal and cubic transitions, similar to those in coarse BT ceramics, were revealed by in-situ temperature dependent Raman spectrum. The multiphase coexistence and the diffused phase transition character were demonstrated in the 8 nm nanocrystalline BT ceramics. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2002CB613301) and the National Natural Science Foundation of China (Grant No. 50872093)     

Phase Evolution and Oxidation Resistance of YSZ/(Ni,Al) Composite Coatings in CH<Subscript>4</Subscript> Atmosphere

YSZ/(Ni, Al) composite coatings with different Ni:Al mole ratios were deposited on superalloy Inconel 600 by electrophoretic deposition (EPD) technique, followed by sintering in CH₄ atmosphere at 1 100 °C for 2 h and isothermally oxidation at 1000 °C for 50 h. After sintering at 1100 °C for 2 h in CH₄ atmosphere, besides ZrC and t-ZrO₂ phases, the phase constitutes of Ni:Al mole ratios with 1:3, 1:2, and 1:1 were (Zr, Al)C, AlNi₃ and Ni phases, respectively. A remarkable difference in the oxidation behaviors of YSZ/(Ni, Al) composite coatings with different Ni:Al mole ratios was observed. For YSZ(Ni:Al=1:3) coated sample, oxidation at 1000 °C causes decomposition of the (Zr,Al)C solid solution to metallic Al, and then most of the Al is oxidized to Al₂O₃. For the YSZ(Ni:Al=1:2) coated sample, oxidation at 1000 °C mainly causes decomposition of the AlNi₃ phase. For YSZ(Ni:Al=1:1) coated sample, after oxidation at 1000 °C, most of the Ni is oxidized to NiO phase, and tolerated 50 h of oxidation and finally cracked and spalled from the specimen. YSZ(Ni:Al=1:3) and YSZ(Ni:Al=1:2) coated samples show superior oxidation resistance than that of YSZ coating. The different oxidation resistance mechanisms of YSZ/(Ni, Al) composite coatings sintered in CH₄ atmosphere were discussed.     

Enhanced Low-field Magnetoresistence in the Absence of Mn Content in Polycrystalline La0.67Ca0.33MnO3

Magnetic and electrical transport properties of the La0.67Ca0.33Mn1-xO3 （x=0-0.16）, which were prepared by the sol-gel method followed by sintering treatment at 1 450, 1 100 and 900 ℃, respectively, were investigated. Experimental results show that, with the increase of x, the resistivity of samples increases and the insulator-metal transition temperature shifts towards lower temperature. Meanwhile, the intrinsic megnetoresistance effect is weakened and the extrinsic magnetoresistance is enhanced. For the samples with x=0.16 and 0.10 sintered at 1 100 ℃ and 900 ℃, respectively, low field magnetoresistance as high as about 50% can be observed. Furthermore, for the samples sintered at 1 100 ℃ and 900 ℃, the grain size is not only controlled by about sintering temperature, but also by the absence of Mn content x.     

Investigation on phase evolution of the ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass ceramics

ZnO-B₂O₃-SiO₂-Al₂O₃-Na₂O glass doped with nucleating agent TiO₂ was prepared with melting-quenching method and the effect of nucleating agent on the crystallization behavior and phase evolution of this glass was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The experimental results show that the glass transition temperature and the first crystallization temperature decrease from 630 °C and 765 °C to 595 °C and 740 °C, respectively, with introduction of TiO₂ into glass. There is no diffraction peaks in the XRD pattern but it is no longer transparent for the base glass without nucleating agent after heat treatment, which suggests the serious phase separation occurred, and the observation by SEM indicates that the phase separation is developed by nucleation and growth mechanism. However, there are two different crystals ZnAl₂O₄ and NaAlSiO₄ present in the glass containing TiO₂ after heat treating at 575 °C for 2 h and 740 °C for 6 h, respectively. What is interesting is that NaAlSiO₄ disappears as the crystallization time at 740 °C increases from 6 h to 12 h, and more ZnAl₂O₄ crystal is formed, namely, the further formation of ZnAl₂O₄ is at cost of NaAlSiO₄ with increasing crystallization time. And observation of the morphology of glass ceramics shows great difference with increasing crystallization time. Moreover, the ability of ZnO-B₂O₃-SiO₂-Al₂O₃-Na₂O glass ceramics against attacking of 1M HCl solution is increased by the crystals precipitated in heat treatment process.     

Effect of the silicon powder content on the properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiC-Me composites

By means of transient plastic phase process, the Al₂O₃-SiC-Me composites were produced throungh adding metal aluminium and silicon-powder to Al₂O₃-SiC materials. Under the condition of the same content of silicon and aluminium mixed-powder, the effect of silicon powder addition on properties of Al₂O₃-SiC-Me composites was studied by means of XRD and EPMA analyses in the temperature range of 300°–1600°C. The results indicated that the content of metal phase in the sample at 1600°C increases with increasing silicon powder content. At the same time, when the temperature is lower than 1100°C, the strength of samples gradually increases with increasing temperature. However when the temperature is higher than 1100°C, the strength of samples gradually decreases with increasing temperature, and this change is very small. The results also indicated that at the same sintering temperature, the sample with 6% silicon powder has the maximum strength. Supported by the Chen Yunbo Academician Fund (Grant No. 2007CY021)     

Effect of temperature on polishing and flatting time of float-formed Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass

Li₂O-Al₂O₃-SiO₂ glass prepared from traditional melting and cooling process was investigated. The surface characteristic of LAS glass was studied by simulating tin bath with different temperature and time, and the effect of surface tension/viscosity ratio on flatting time was analyzed. The results demonstrated that LAS glass can polish effectively when polishing at 1 300 for 8 °C min, and the optimum flatting and polishing temperature was 1 250–1 300 °C.     

Synthesis and sintering of Mg<Subscript>2</Subscript>Si thermoelectric generator by spark plasma sintering

Raw Mg,Si powder were used to fabricate Mg2Si bulk thermoelectric generator by spark plasma sintering （SPS）.The optimum parameters to synthesize pure Mg2Si powder were found to be 823 K,0 MPa,10 min with excessive content of 10wt% Mg from the stoichiometric Mg2Si.Mg2Si bulk was synthesized and densified simultaneously at low temperature （823 K） and high pressure （higher than 100 MPa） from the raw powder,but Mg,Si could not react completely,and the sample was not very dense with some microcracks on the surface.Then,Mg,Si powder reacted at 823 K,0 MPa,10 min in SPS chamber to form Mg2Si green compact,again sintered by SPS at 1023 K,20 MPa,5 min.The fabricated sample only contained Mg2Si phase with fully relative density.